Methods of producing ceramic or glass-ceramic articles are well known. The economies associated with conventional part production generally favor the production of large quantities of identical parts. Corresponding methods most of the time require complex processes involving the production of molds or the like. For commercial reasons, these production methods are generally unacceptable for small quantities. For prototyping, the production of small series of ceramic or glass-ceramic articles or the production of a plurality of articles each having different shape, only few economically viable methods are know. Mostly such methods involve a subtractive machining method. In such subtractive machining methods material is cut away from a starting block of material to produce a more complex part. Examples of such methods include: milling, drilling, grinding, lathe cutting, flame cutting, use of an electrical discharge machine etc. The problems associated with these production methods are large initial investment, waste of material and tool wear. The latter is costly and reduces accuracy of the articles produced. One method to produce shaped ceramic or glass-ceramic articles is computer controlled milling of shaped bodies out of the solid ceramic or glass-ceramic material, which inevitably leads to considerable waste that has to be reprocessed at great effort and expense. Also, complicated shapes with deep hollows are not accessible by the aforementioned methods or require complex procedures.
In order to solve these problem free form fabrication methods (solid free form fabrication methods) have been developed. This term relates to a collection of methods that have been successfully applied to produce articles from materials such as metals, plastics, ceramics, and the like. Typical examples are (i) electron beam melting, which involves melting of metal powder and produces substantially void free articles without pores, (ii) laser engineered net shaping, wherein a laser is used to melt metal powder and to deposit it on a substrate and (iii) selective laser sintering (SLS) or selective laser melting (SLM), which uses a laser to fuse powdered nylon, elastomer, metal or ceramic material. SLM usually requires additional processing to produce fully dense parts. For a typical example of a freeform fabrication method see also Griffith et al. “Free form fabrication of metallic components using laser engineered net shapings”, Solid free form fabrication symposium, Austin, Tex., Aug. 12-14, 1996.
The automatic construction of physical objects using free form fabrication is called rapid prototyping. Rapid prototyping takes virtual designs from computer aided design (CAD) or animation modeling software or other data that describe the shape of physical objects, transforms them into data of thin, virtual, horizontal cross-sections of said physical objects and builds a new physical object on the basis of the data of the cross-sections.
An apparatus lays down successive layers of liquid, powder, or sheet material, and in this way builds up the new object from a series of said cross-sections. Selective laser melting (SLM) is a special type of rapid prototyping, wherein layers of material (in particular layers of powdered material) are molten and joined by subsequent crystallization. Aspects of rapid prototyping have for example been described in D. T. Pham, S. S. Dimov, Rapid manufacturing, Springer-Verlag, 2001, ISBN 1-85233-360-X.
U.S. Pat. No. 4,863,538 discloses a method of producing an article from plastic, metal or ceramic material by a free form sintering method, i.e. by sequentially sintering a plurality of powder layers to build the desired part in a layer-by-layer fashion. Heating is effected by means of a laser. EP 0 946 325 B1 discloses “Selective Laser Melting” (SLM). Various other publications have been published concerning this type of technology. However, ceramic or glass-ceramic articles produced by such known methods often have a fairly large amount of cracks, fissures and other imperfections, and therefore have inferior properties, especially inferior mechanical properties, inferior biocompatibility, high porosity and the like and are therefore not suited for sophisticated applications, e.g. as dental article or in certain applications in the electronic industry.
The often inferior properties of ceramic or glass-ceramic articles produced by such methods are primarily a result of the very high melting points of ceramic materials. These melting points make rapid rates of heating and cooling of the material necessary and result in large temperature differences among different parts of the articles produced during the production process. This leads to high stresses inside the ceramic or glass-ceramic articles which in turn lead to cracks, fissures and other defects.
Various means to improve the properties of ceramic or glass-ceramic articles have been proposed. U.S. Pat. No. 5,508,489 proposes the use of at least two laser beams to heat the powder, wherein one laser is used to sinter the powder and a defocused laser is used to provide a predetermined temperature gradient between the sintering location and the surrounding powder. This method reduces curling of the produced layers.
U.S. Pat. No. 7,452,500 describes a method, wherein a high-energy beam irradiates predetermined positions of a powder layer a plurality of times, wherein each position is first at least once heated to a temperature below the melting point of the powder material and during the second or a later irradiation heated to a temperature above the melting temperature. In SLM a layer of material is usually heated by means of laser irradiation on only one side of the layer, the side of the layer facing away from the beam is heated solely by heat transfer within the layer, which may be slow compared to the heating rate. A high temperature gradient between the two sides of the layer is the result. This may lead to evaporation, in particular explosive evaporation, on the side of the layer heated by the beam, while the side of the layer facing away from the beam may not even be molten.
U.S. Pat. No. 7,452,500 solves this problem by heating during a plurality of intervals. During the waiting period between two intervals the temperature gradient between the two sides of the layer is reduced by heat transfer, while the beam heats other regions of the layer. Thus the method avoids large temperature differences between the two sides of the layer to be melted, which avoids the risk of evaporation or even explosive evaporation of layer material due to overheating and reduces the time required to make an article. It also leads to reduced stress. An apparatus for free form fabrication is also disclosed in U.S. Pat. No. 7,452,500.
Reference is further made to the following documents:    U.S. Pat. No. 4,814,575 AUS 2006/119017 A1    EP 1 561 839 A1    U.S. Pat. No. 5,393,482 A    U.S. Pat. No. 6,767,499 B1    JP 2003 001368 A    DE 10 2004 041 633 A1    U.S. Pat. No. 4,863,538 A1    WO 2005/095304 A1    WO 0 240 744    EP 0 129 188    EP1 772 210    WO 2004/089 851    DE 10 2005 048 314    SHISHKOVSKY ET AL: “Alumina-zirconium ceramics synthesis by selective laser sintering/melting” APPLIED SURFACE SCIENCE, ELSEVIER, AMSTERDAM, NL, Vol. 254, No. 4, 23. Nov. 2007, Pages 966-970,    MERINO ET AL: “Ionic conductivity in directionally solidified Al2O3-ZrO2(3% mol Y2O3) near eutectic composites” SOLID STATE IONICS, NORTH HOLLAND PUB. COMPANY. AMSTERDAM, NL, Vol. 178, No. 3-4, 6. Mar. 2007, Pages 239-247    WANG A H ET AL: “Microstructural characteristics of Al2O3-based refractory containing ZrO2 induced by CO2 laser melting” APPLIED SURFACE SCIENCE ELSEVIER NETHERLANDS, Vol. 221, No. 1-4, 15. Jan. 2004, Pages 293-301    BOURBAN ET AL: “Solidification microstructure of laser remelted Al2O3-ZrO2 eutectic” ACTA MATERIALIA, ELSEVIER, OXFORD, GB, Vol. 45, No. 12, 1. Dec. 1997, Pages 5069-5075    LARREA A ET AL: “ZrO2-Al2O3 eutectic plates produced by laser zone melting” JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, ELSEVIER SCIENCE PUBLISHERS, BARKING, ESSEX, GB, Vol. 22, No. 2, 1. Feb. 2002, Pages 191-198,    LLorca et al: Progress in Materials Science 51 (2006) Pages 711-809.
However, until to date no method of free form fabrication is known for the preparation of ceramic or glass-ceramic articles with similar or even better material properties than those produced by the above mentioned subtractive machining methods.